Tree highlight selected items on hover

[havi05]

Hovered items of a Tree are now highlighted, even if they are already selected.
Equivalent to this pr #99361, but for the Tree node.
Relates to godotengine/godot-proposals#7404

[Calinou]

Tested locally, it works as expected. I made sure no unnecessary redrawing occurs.

tree_hover_editor.mp4

tree_hover_project.mp4

However, when using a light editor theme, hover text becomes white when it should remain black:

tree_hover_editor_light.mp4

[havi05]

Is fixed in editor_theme_manager.cpp line 960.